1. Field of the Invention
The present invention relates to a webbing winding device which structures a seat belt apparatus of a vehicle or the like, and to a clutch mechanism used for switching between transmission and disconnection of driving force of a motor or the like.
2. Description of the Related Art
A seat belt apparatus, which restrains a body of an occupant sitting at a seat of a vehicle with a long belt-form webbing belt, is equipped with a webbing winding device fixed to a body of the vehicle sideward of the seat. The webbing winding device is equipped with, for example, a spool (winding shaft) whose axial direction is substantially along a front-rear direction of the vehicle. A length direction proximal end side of the webbing belt is anchored at this spool. The spool accommodates the webbing belt in a state in which the webbing belt is wound on at an outer peripheral portion of the spool in a layered form.
An urging member such as a torsion coil spring or the like is provided at the webbing winding device, and urges the spool in a winding direction for winding up the webbing belt. The webbing belt is wound on and accommodated by urging force of this urging member, and slackness or the like of the webbing belt in a state in which the webbing belt is applied to the body of the occupant is eliminated by the urging force of the urging member.
This type of webbing winding device is a structure at which a tongue plate is provided at a length direction intermediate portion of the webbing belt. When the tongue plate is pulled, against the urging force of the urging member, the webbing belt that has been wound on at the spool is drawn out. In this state, the tongue plate is retained at a buckle device which is provided sideward of the seat, at a side thereof opposite to the side at which the webbing winding device is disposed. Thus, the webbing belt can be applied to the occupant.
Further, mechanisms have been considered in which, at a time of rapid deceleration of the vehicle or the like, for the purpose of more strongly restraining the body of the occupant, for example, a driving force of a motor is applied to the spool at the time of rapid deceleration of the vehicle or the like. The spool is rotated in the winding direction by a certain amount by this driving force, and raises tension of the webbing belt.
In this type of webbing winding device, if the spool and a drive shaft of the motor are simply coupled (in the mechanical terms) at usual times, such a coupling becomes a hindrance to rotation of the spool when the webbing belt is drawn out and wound up for usual purposes, and unnecessary loads may be applied to the motor. Accordingly, a clutch mechanism is provided between the drive shaft and the spool.
Further, in a structure which, as described above, supplies driving force of the motor to the spool and winds up the webbing belt with this driving force at a time of rapid deceleration of the vehicle, if a rotation amount of the motor is large, and rotation of the spool in the winding direction is excessive, the body of the occupant will be constricted by excessive force from the webbing belt.
This state, in which the webbing belt constricts the body of the occupant more than necessary while the webbing belt is applied, is a state in which rotation of the spool in the winding direction is hindered by the body of the occupant. Accordingly, employment of a “torque limiter”, which releases the mechanical coupling between the motor and the spool in a state in which the rotation of the spool is hindered, when a rotational torque applied to the spool from the motor is at or above a predetermined magnitude, has been considered as one means for preventing this excessive constriction.
Now, reductions in size and weight of devices mounted at vehicles, such as webbing winding devices, are constantly being demanded. However, when a torque limiter is provided in addition to a centrifugal clutch as described above, weight of the webbing winding device is raised and the webbing winding device becomes larger.